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Garage and root cellar insulation

I received a call from my mechanical engineer today asking me how I planned to insulate the garage and root cellar ceilings. Both rooms are in the basement and above those ceilings are first floor living spaces. I told him my plan is to dense pack them with cellulose and probably add an inch of Polyiso covered with a layer of gypsum to break the thermal bridge and satisfy fire code. I was planning on insulating the rim joists with a couple of inches of ccSF on the inside, the only area in the house with the nasty stuff, mineral wool on the outside, and then dense pack would fill the bays.

Anyone who has ever lived in a house with a garage below a living space knows how problematic this detail can be. I once lived in a house with a bedroom above the garage and it was always cold in the winter. In our attempt to fix the issue, we discovered a few issues with the way the way the “builder” insulated the space. First is there was plumbing going down the middle of one of the joist bays and they just stuffed R38 batts between the PEX and the gypsum, so in a 12′ deep joist bay, the R38 covered only the bottom half of the bay. If this was only in the middle of the garage, maybe not so bad, but there was a tub next to an exterior wall and so half the rim joist was uninsulated and well as half the joist bay. Next there were two bathroom exhaust vents going through the bays so a nice 4″ aluminum flex duct providing a thermal bridge the entire length of 2 joist bays, with the same compressed R38 issue. The obvious fault was the thermal bridge of the joists themselves. Finally, the R38 when fitted in the I-joists created a gap around both the top and bottom cord. Since the rim joists were not separately insulated, we had an uninsulated section of the floor above open to an uninsulated section of the rim joist that provided a path for warm air to escape.

I think the detail I am contemplating for the new house overcomes those issues because:
1) No mechanicals in those ceiling sections to mess up my insulation, 2) Completely sealed and insulated rim joists and 3) thermal break provided by the Polyiso, There is the concern that DP will settle over time, but if the space is properly insulated below and in the rim joists, then that air gap should not really be issue, right? Any other thoughts on how to better insulate these spaces?


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  1. GBA Editor
    Martin Holladay | | #1

    Your details sound good. For more information on the topic, see my article: How to Insulate a Cold Floor.

    I'll take this opportunity to note that when a garage is located under living space, it's extremely important to do an impeccable job of air-sealing the ceiling. The quality of the air in a garage is often nasty, and you don't want your garage air to pollute the indoor air upstairs.

  2. Steve Knapp CZ 3A Georgia | | #2

    Jonathan. Are you planning to use this same approach for the root cellar?

  3. Jonathan Lawrence CZ 4A New Jersey | | #3


    Thanks for the response and the article. Funny that is shows floor trusses as that is what I am planning on using throughout the house, except in those areas where we need to dense pack, so the garage ceiling, root cellar, and the flat roof sections. The trusses are not cheap, and replacing them with closed web joists or dimensional lumber in those areas saves me some money. After reading the article, I think I am going to add 3" of open cell spray foam to the underside of the subfloor, just to be safe. I was planning on using Solitex Mento air barrier between the rim board and the joists and then tape it to the Polyiso to create a continuous air barrier. Since there is no electrical or mechanical in the ceiling, the gypsum will act as a second air barrier.


    I was planning on detailing the root cellar the same way, but your question made me think I was missing something. The root cellar is in the NE corner of the house and will have two outside poured concrete walls. There will be an interior poured concrete wall that separates the root cellar from a wine cellar. The fourth wall is a 2x6 stud wall with an insulated door. The will be a 4" PVC pipe coming in on the north side, penetrating the rim joist (we are only 15 inches above grade at this corner) and terminating near the floor. Diagonal to this intake vent will be an exit vent penetrating the rim joists on the east side. I was not going to put a vapor barrier or insulation under the concrete floor in the root cellar or the garage. The wine cellar will have both, as will the rest of the basement. I am also planning on putting one sump pump in the root cellar. If I put it in there, I don't have to worry about insulating it and it should provide some additional humidity. I confirmed with my plumping inspector that I can put it in there. All of the foundation walls will be waterproofed, so the floor is my only natural source of humidity. The basement walls are insulated from the inside, so I will skip that in the root cellar.

    As I compare the expected ambient conditions between the garage and root cellar, humidity is the biggest difference. I don't want 90% humidity from the root cellar getting into a 12" joist bay with DP cellulose and open cell spray foam. Despite being vapor open, just seems a bit risky and I would rather keep the humidity in the root cellar. So I would change the root cellar Polyiso to foil faced. I guess it would not hurt to do the same in the garage. Is this why you asked, or are you thinking of something else?



  4. Dennis Heidner | | #4

    Jonathan, you're probably not going to like my answer...however - garages under living spaces can be a problem. I have one. After seeing one car fire (a vehicle I previously owned and sold two weeks before to a friend)... I no longer park my vehicles in the garage. Again - its because its under bedrooms. If it have been a semi-detached or detached garage... I'd feel better.

    It's hard to keep the bad stuff (VOC's & CO ) that you can get with garages from making it into the bedrooms. You may want to think carefully about air sealing for that more than air sealing for heat loss. The garage will need a fresh air/ventilation of its own - which probably means that it will be colder than the rest of the house. It could be at nearly the same outside temperature - so insulate it accordingly. I would use Roxul anywhere around the garage that I can... It can handle fires and offer up a much better resistance than the spray on foam even when the foam has fire retardants included or sprayed on them.

    Twenty years ago - I used to think the garage in the house was good... now I am definitely in the crowd that tries to find another way...

  5. Steve Knapp CZ 3A Georgia | | #5

    Jonathan. I've read about designs for dry (canned goods) and wet (vegetable) root cellars. I'm assuming you want to have the latter.

    My concern is with the high humidity levels you typically need to successfully store vegetables (70 to 90 percent). Any materials you use in that area will have to be able to handle a moist environment.

  6. Jonathan Lawrence CZ 4A New Jersey | | #6


    Thanks for the input and sorry to hear about the fire. In the current design, the garage is below the family room. The idea was to get a full first floor of living space. CO is still a concern for me as I have one electric car and one gas car. Even if I replace the gas car with electric in a couple of years, CO concern goes away, but not fire, specifically electrical fire. The way I plan to deal with CO in the short-term is two-fold. First, is by designing the ceiling to be air-tight. Between the Mento, taped Polyiso and taped gypsum, I should be able to make it a very air tight structure. In addition, the DP cellulose, while not an air barrier, does a good job of reducing air infiltration. The family room will have both smoke and CO detectors, plus I will add a plug-in CO detector. In my current house, we have both ceiling mounted CO and smoke detectors as required by code and I added plug-in CO detectors low to the ground in the bedrooms as an insurance policy. As for fire prevention, there will be few items in the garage that will burn. 3 of the walls are poured concrete, the door between the basement and garage will be aluminum clad and the garage doors will be steel clad with a foam core. We needed some large beams for structural support and I asked my structural engineer to spec wood over steel if possible to avoid thermal bridging so there will be parallams that extend into the garage, but they tend to char not burn. The garage will also have a smoke detector. I am using the Roxul ComfortBoard 80 or 110 for the exterior insulation on the house and I could use it for the garage ceiling. However, if I do so, I lose the air barrier benefit of the Polyiso, plus some R-value. I would not expect the garage temp to get much below 40, so the Polyiso should give me its rated R-value. As for VOC's, I don't really have any. The paints I use nowadays are all low to no-VOC, water-based, and no need for solvents or minerals spirits. My snow-blower is electric. My only combustion tools are leaf blower and chainsaw. I rarely have more than a gallon of mix on site at any time.

    This scenario is in direct contrast to the house I grew up in. My Dad was a mechanical engineer who worked in munitions when he was in the army. Our garage and dirt cellar were full of gun powder - he made is own bullets, lead - he made his own sinkers, acetylene and oxygen tanks - he loved to weld and cut a steel, chemicals - he worked for a chemical company, oil-based paints, you name it. And everything was kept in the garage that was below the kitchen and dining room. It was also a heated garage, heated by the same oil-burning furnace that heated the rest of those! It is amazing how different things are today.


    Yes, the latter. Thanks for the reminding me that I need to protect the ceiling above the root cellar from the high humidity. I am also going to address the humidity in the wine cellar too. It is typical for actively cooled wine cellars to have 5" of ccSF in the ceiling and 3" in the walls as you want to keep the humidity levels relatively high, around 70%. The problem is the bottom half of the joist and interior portion of the studs then live in this high humidity environment. Does not seem like a good idea long-term. I can use a Siga low-perm membrane attached to the bottom chord of the joists, then foil-faced Polyiso. Seems like a appropriate stack-up for the interior stud walls too, although I would skip the SF and just use DP in the bays. The interior sheathing will be a bamboo product. Not sure if it makes sense to add a vent channel between the bamboo and the Polyiso, although I guess it can't hurt.



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